702 research outputs found
Dynamics stabilization and transport coherency in a rocking ratchet for cold atoms
Cold atoms in optical lattices have emerged as an ideal system to investigate
the ratchet effect, as demonstrated by several recent experiments. In this work
we analyze theoretically two aspects of ac driven transport in cold atoms
ratchets. We first address the issue of whether, and to which extent, an ac
driven ratchet for cold atoms can operate as a motor. We thus study
theoretically a dissipative motor for cold atoms, as obtained by adding a load
to a 1D non-adiabatically driven rocking ratchet. We demonstrate that a current
can be generated also in the presence of a load, e.g. the ratchet device can
operate as a motor. Correspondingly, we determine the stall force for the
motor, which characterizes the range of loads over which the device can operate
as a motor, and the differential mobility, which characterizes the response to
a change in the magnitude of the load. Second, we compare our results for the
transport in an ac driven ratchet device with the transport in a dc driven
system. We observe a peculiar phenomenon: the bi-harmonic ac force stabilizes
the dynamics, allowing the generation of uniform directed motion over a range
of momentum much larger than what is possible with a dc bias. We explain such a
stabilization of the dynamics by observing that a non-adiabatic ac drive
broadens the effective cooling momentum range, and forces the atom trajectories
to cover such a region. Thus the system can dissipate energy and maintain a
steady-state energy balance. Our results show that in the case of a
finite-range velocity-dependent friction, a ratchet device may offer the
possibility of controlling the particle motion over a broader range of momentum
with respect to a purely biased system, although this is at the cost of a
reduced coherency
Current reversals in a rocking ratchet: dynamical vs symmetry-breaking mechanisms
Directed transport in ratchets is determined by symmetry-breaking in a system
out of equilibrium. A hallmark of rocking ratchets is current reversals: an
increase in the rocking force changes the direction of the current. In this
work for a bi-harmonically driven spatially symmetric rocking ratchet we show
that a class of current reversal is precisely determined by symmetry-breaking,
thus creating a link between dynamical and symmetry-breaking mechanisms
Inhibition of electromagnetically induced absorption due to excited state decoherence in Rb vapor
The explanation presented in [Taichenachev et al, Phys. Rev. A {\bf 61},
011802 (2000)] according to which the electromagnetically induced absorption
(EIA) resonances observed in degenerate two level systems are due to coherence
transfer from the excited to the ground state is experimentally tested in a
Hanle type experiment observing the parametric resonance on the line of
Rb. While EIA occurs in the transition in a cell
containing only vapor, collisions with a buffer gas ( of )
cause the sign reversal of this resonance as a consequence of collisional
decoherence of the excited state. A theoretical model in good qualitative
agreement with the experimental results is presented.Comment: 8 pages, 7 figures, submitted to Physical Review
Storage and retrieval of light pulses in atomic media with "slow" and "fast" light
We present experimental evidence that light storage, i.e. the controlled
release of a light pulse by an atomic sample dependent on the past presence of
a writing pulse, is not restricted to small group velocity media but can also
occur in a negative group velocity medium. A simple physical picture applicable
to both cases and previous light storage experiments is discussed.Comment: 4 pages, 3 figures, submitted to Physical Review Letter
Realisation and Characterisation the BEC for 87Rb Atoms
تكاثف بوز- انشتاين هي حالة من حالات المادة يتم خلقها عن طريق تبريد ذرات غاز لدرجة حرارية فائقة الأنخفاض وبكثافة منخفضة جداً. تم توليد تكاثف بوز- انشتاين الى ذرات 87Rb عن طريق المصيدة المغناطيسية بترتيب يطلق عليه Quadrupole –Ioffe Configuration الذي يتكون من مجال مغناطيسي رباعي يتولد عن رتيب زوج من الملفات بترتيب يسمى Anti-Helmholtz coils . كما يستخدم سلك منفرد اضافي يولد مجال عمودي على المجال السابق يطلق عليه Offset coil والغرض منه هو كبح انتقالات ماجورانا Majorana flip-flop . اول خطوات العمل هو تبريد وقنص الذرات باستخدام الليزر في المصيدة المغناطيسية الضوئية Magneto-Optical Trap وصولاً الى عدد ذرات بحدود ذرة بزمن 3 ثواني. بعد ذلك يتم تبريد الذرات في ميكانيكية Optical Molasses للوصول الى درجة حرارة أقل وتحقيقاً لكثافة ذرية عالية. الخطوة التالية هوتحميل الذرات الى المصيدة المغناطيسية بزمن عمر قدره وهو كافي لتبريد الذرات لدرجة حرارية اقل لعملية التبخير Evaporative cooling ومن ثم انتاج ال BEC . يتم التبريد بالتبخير باستخدام ترددات راديوية للوصول الى انتقال الطور الى BEC . تم قياس درجة حرارة الذرات باستخدام تقنية زمن الطيران Time Of Flight وان سرعة تمدد الغيمة الذرية قيست بمقدار والتي تودي الى قياس درجة الحرارة بمقدار and وهي مقاربة الى درجة حرارة الارتداد Recoil temperature للربيديوم.The Bose-Einstein condensate (BEC) is created in a magnetic trap in the Quadrupole-Ioffe configuration (QUIC). This kind of trap combines an anti-Helmholtz quadrupole field with an offset field produced by a single coil perpendicular to the quadrupole field axis to suppress Majorana transitions. In the quadrupole trap evaporative cooling is performed by using radio frequency, reaching the phase transition to a BEC in the QUIC trap. By using Time of Flight (TOF) technique, the expansion velocity is measured with and which lead to temperature of and It is roughly around the recoil temperature.
 
Current reversals in a rocking ratchet: the frequency domain
Motivated by recent work [D. Cubero et al., Phys. Rev. E 82, 041116 (2010)],
we examine the mechanisms which determine current reversals in rocking ratchets
as observed by varying the frequency of the drive. We found that a class of
these current reversals in the frequency domain are precisely determined by
dissipation-induced symmetry breaking. Our experimental and theoretical work
thus extends and generalizes the previously identified relationship between
dynamical and symmetry-breaking mechanisms in the generation of current
reversals
Vibrational mechanics in an optical lattice: controlling transport via potential renormalization
We demonstrate theoretically and experimentally the phenomenon of vibrational
resonance in a periodic potential, using cold atoms in an optical lattice as a
model system. A high-frequency (HF) drive, with frequency much larger than any
characteristic frequency of the system, is applied by phase-modulating one of
the lattice beams. We show that the HF drive leads to the renormalization of
the potential. We used transport measurements as a probe of the potential
renormalization. The very same experiments also demonstrate that transport can
be controlled by the HF drive via potential renormalization.Comment: Phys. Rev. Lett., in pres
Cold atom realizations of Brownian motors
Brownian motors are devices which "rectify" Brownian motion, i.e. they can
generate a current of particles out of unbiased fluctuations. Brownian motors
are important for the understanding of molecular motors, and are also promising
for the realization of new nanolelectronic devices. Among the different systems
that can be used to study Brownian motors, cold atoms in optical lattices are
quite an unusual one: there is no thermal bath and both the potential and the
fluctuations are determined by laser fields. In this article recent
experimental implementations of Brownian motors using cold atoms in optical
lattices are reviewed
Temporal build-up of electromagnetically induced transparency and absorption resonances in degenerate two-level transitions
The temporal evolution of electromagnetically induced transparency (EIT) and
absorption (EIA) coherence resonances in pump-probe spectroscopy of degenerate
two-level atomic transition is studied for light intensities below saturation.
Analytical expression for the transient absorption spectra are given for simple
model systems and a model for the calculation of the time dependent response of
realistic atomic transitions, where the Zeeman degeneracy is fully accounted
for, is presented. EIT and EIA resonances have a similar (opposite sign) time
dependent lineshape, however, the EIA evolution is slower and thus narrower
lines are observed for long interaction time. Qualitative agreement with the
theoretical predictions is obtained for the transient probe absorption on the
line in an atomic beam experiment.Comment: 10 pages, 9 figures. Submitted to Phys. Rev.
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